1. Field of the Invention:
This invention relates generally to a control system for an AC induction motor and, more particularly, it relates to a control system for connecting the primary windings of an AC induction motor in a delta-connection at heavy loads and for automatically reconnecting the windings in a wye-connection at light loads to effect energy savings.
2. Description of the Prior Art:
U.S. Pat. No. 1,902,444 issued to Hobart et al on Mar. 21, 1933 discloses an AC motor having stator windings that are connected in either mesh (delta) or star (wye), depending on the magnitude of the load current. By sensing the load current and controlling the winding connections in response to that sensed current, the stator windings may be connected in star or wye for low load current and in mesh or delta for high load current.
In U.S. Pat. No. 2,106,833 issued to W. F. Eames on Feb. 1, 1938, there is disclosed a motor and control system for operating a moving stairway by connecting the primary windings of the motor in a star or wye connection for low speed and changing the windings from the star connection to a delta connection for high speed when needed for carrying passengers. A light-sensitive device is utilized to detect the presence of persons for controlling the stairway to operate at the low and high speeds.
U.S. Pat. No. 2,709,775 issued to A. Del Carlo on May 31, 1955, discloses an automatic regulation system for a motor in which the motor windings are switched from delta to star connection only for such load variations which are maintained for a certain length of time. The switching operation is in response to the amount of current supplied to the motor which corresponds to a predetermined load condition.
In U.S. Pat. No. 3,863,741 issued to Y. Mitsuishi on Feb. 4, 1975, there is shown a control system for reel assembly winding and unwinding a power supply cable of an electrically powered earth-moving machine which includes a three-phase induction torque motor mechanically driving the reel assembly. The torque motor is selectively connected to an electric source by a wye-connection when the machine moves forward and by a delta-connection when the machine moves backward. A shift lever is utilized for changing the moving direction of the machine which is connected to the gear box.
U.S. Pat. No. 4,084,406 issued to D. A. Brenneman on Apr. 18, 1978, shows a solid state starter for a chiller compressor motor which includes a motor current sensing circuit which controllably switches a bank of silicon-controlled rectifiers to conduct current to the motor for starting and running.
In various applications of machines, such as centrifugal turbocompressors, driven by AC induction motors, the operation of the motors is subjected to a wide range of load conditions. It is known that such motors can be constructed to operate with high efficiency and power factor values during full load conditions, but when the motors are operated at reduced or light load conditions the efficiency and power factor are substantially decreased thus causing a waste of energy. For optimum operation during normal or full load conditions, the motor windings are usually connected in delta. In accordance with the present invention, it has been found that significant improvements in efficiency and power factor can be achieved by reconnecting the primary windings of such AC motors in a wye-connection, for coupling to the electric source, in response to the power delivered to the motor load dropping below a preselected level.